Endogenous peroxidase- and vimentin-positive cells accumulate at the corticomedullary border of the chicken thymus

The granulopoietic activity of the thymus and bursa of Fabricius was analyzed using as anti-vimentin monoclonal antibody and endogenous peroxidase reaction to detect possible hemopoietic precursors and functionally mature cells, respectively. Endogenous peroxidase-positive cell (EPC) and vimentin-positive cells (V9) were located at the corticomedullary zone of the thymus. The cells were grouped, suggesting their clonal expansion. The first EPC appeared in the thymus on Day 15 of embryonic age, 2 and 3 days later than in the bursa of Fabricius and spleen, respectively. They were intermingled with thymocytes. Although the bursal EPC disappeared by Day 7 after hatching, the number of thymic EPC continued to increase. The thymic EPC and V9 cells persisted for more than 12 wk of age. The V9 cells were present either as single cells or grouped cells in the 15-day-old embryos. The V9 cells represented a distinct cell population of the corticomedullary border that may cooperate with the EPC